COLE. — IMAGE- FORMING POWERS OF VARIOUS TYPES OF EYES. 403 



however, important to them, since it aids them in remaining concealed, 

 and is thus of service in protecting them from enemies to which they 

 would be exposed if they came into the light, and in preventing them 

 from going into situations that would be unsuitable in other respects 

 (lack of moisture, etc.). The negative reaction to light is not, however, 

 the only form of response to change in environmental conditions which 

 operates to keep these animals in their accustomed habitats ; responses 

 to contact stimuli, to moisture, temperature, and other variable condi- 

 tions, are often undoubtedly of as much, if not, indeed, of greater impor- 

 tance. Such, for example, must be the case, as pointed out by Loeb 

 ('90, p. 51), in the larva of the willow-borer, which lives in a dark 

 situation, but is, nevertheless, positive in its reaction to light when 

 exposed to its influence. It is not necessary to enlarge upon the 

 advantages which the ability to perceive distant objects (i. e., those 

 with which the oganism does not come in direct contact) in their 

 proper spacial relation gives to animals living in the light. An organ- 

 ism, by means of what we ordinarily speak of as " sight," is brought 

 into close relation with a part of its environment which organisms 

 wanting this faculty have no means of appreciating. This obviously 

 is of distinct advantage. As the converse of what has just been said 

 about dark-inhabiting forms, it may be expected that those normally 

 living in the light are as a rule positively phototropic, and this seems 

 in general to hold true. Where a change in the character of the pho- 

 totropic response takes place, — as is the case in many animals which 

 are under certain conditions positive and under others negative, — it 

 is probable that this change can in most cases be explained by the 

 natural habits of the animal. Different factors may operate to bring 

 about this change of reaction ; it may be mechanical stimulation, a 

 variation of temperature, or a change in the intensity of the light itself. 

 The result, in many cases at least, is to bring the organism into an 

 optimum condition, either as to light intensity or some other stimulus. 

 The frog (Rana), for example, becomes negative to light at tempera- 

 tures below 10° C, and this response is probably an important one 

 under natural conditions in inducing the animal to swim downward 

 to the bottom of the pond and bury itself on the approach of cold 

 weather. 



We do not need to concern ourselves at this point with the theories 

 as to how this adaptation has come about. It is sufficient to show 

 that such adaptation exists. It may be asked whether certain animals 

 have become negatively phototropic because they live in dark places, 

 or whether they live in dark situations because they are negative in 

 their reactions to light. The two probably have developed together. 



